The impact of climate on the hydrogeology and stability of a large excavation in a glacial till

The Department of Regional Development (DRD), Northern Ireland, designed and constructed numerous highways cuttings in stiff overconsolidated glacial tills during the 60's and 70's. The designs were largely based on ~ntemporary empirical approaches using undrained shear strength, with limited consideration given to the long-term stability of the slopes. Recently there has been an increased frequency of reported slope failures including the failure at Dromore, Co. Down. Consequently Queen's University of Belfast and Roads Service (Northern Ireland), in collaboration With the University of Saskatchewan, have completed a comprehensive hydrogeological research study on a new 18m deep cutting in a drumlin formed in overconsolidated till at Loughbrickland, Co. Down; Northern Ireland. The primary objective was to examine the effects of environmental changes on the groundwater regime with a particular focus on the stability of the slope. A laboratory and field instrumentation programme was carried out including detailed monitoring of head levels and local meteorological conditions. Three years of validated climate and head level data provided a comprehensive understanding of seasonal hydrogeological fluctuations within the Loughbrickland drumlin slope. The head level fluctuations demonstrate a clear relationship with seasonal variation in climate. The· data set provides a basis for further investigation into the effects of cyclic pore water pressure fluctuations and progressive failure on the long-term stability ofslopes. The collated field data was used to create a transient coupled seepage/slope stability model, using SeeplW and SlopelW (Geoslope International, Calgary, Alberta) to analyse the impact of climate on pore water pressure and subsequently slope stability. The model was calibrated using the field and laboratory data and was capable of accurately simulating the annual fluctuations in head level. A sensitivity analysis was completed to .assess the effect of predicted UKCIP climate scenarios on the slope stability of the cutting at Loughbrickland. The model simulates that climate change (UKCIP scenarios) will have minimal effect on the annual cycle of head levels and therefore on the short to medium term slope stability.